Solar cells are generally made of semiconductor materials, such as silicon (Si), which convert sunlight into useful electrical energy. A solar cell contact is in generally made of thin wafers of Si in which the required PN junction is formed by diffusing phosphorus (P) from a suitable phosphorus source into a P-type Si wafer. The side of the silicon wafer on which sunlight is incident is generally coated with an anti-reflective coating (ARC) to prevent reflective loss of sunlight. This ARC increases the solar cell efficiency. A two dimensional electrode grid pattern known as a front contact makes a connection to the N-side of silicon, and a coating of predominantly aluminum (Al) makes connection to the P-side of the silicon (back contact). Further, contacts known as silver rear contacts, made out of silver or silver-aluminum paste are printed and fired on the P-side of silicon to enable soldering of tabs that electrically connect one cell to the next in a solar cell module. These contacts are the electrical outlets from the PN junction to the outside load.
The back side of the silicon wafer typically includes Al paste, but generally lacks an ARC. Conventional back-side Al pastes do not fire through typical ARC materials such as SiNx, SiO2, and TiO2. Conversely, pastes that fire through well on the front side of silicon do not form a Back Surface Field (BSF) layer, and are hence unsuitable for use in solar cell back contacts.
Hence, there is room in the art for a back-side paste that can both (1) fire through a passivation layer (SiNx or SiO2 or TiO2) and (2) simultaneously achieve good BSF formation on the back side of silicon.
Presently, a typical solar cell silicon wafer is about 200-300 microns thick, and the trend is toward thinner wafers. Because the wafer cost is about 60% of the cell fabrication cost, the industry is seeking ever-thinner wafers, approaching 150 microns. As the wafer thickness decreases, the tendency toward bowing (bending) of the cell due to the sintering stress increases, which is generated by the great difference in the thermal coefficients of expansion (TCE) between aluminum (232×10−7/° C.@20−300° C.) and silicon, (26×10−7/° C.@20−300° C.).
Known methods of mitigating silicon wafer bowing include reduction of aluminum content during screen-printing that causes incomplete formation of BSF layers and requires a higher firing temperature to achieve the same results. Chemical (acid) etching has been used to remove the Al—Si alloy that forms after firing the Aluminum paste. This is just another step in the manufacturing process that leads to additional cost.
Another approach is to use additives to reduce the thermal expansion mismatch between the Al layer and the silicon wafer. However, a drawback is a reduction in back surface passivation quality and a concomitant reduction in solar cell performance. Partial covers, where only a portion of the back side of the wafer is coated with aluminum, have been used to form a BSF to counteract bowing, which causes a reduction in cell performance.
Finally, another conventional way to reduce or eliminate bowing is cooling a finished solar cell from room temperature to ca. −50° C. for several seconds after firing. With such plastic deformation of the Al—Si paste matrix, bowing is largely eliminated, but this represents an additional process step, and there is a high danger of breakage from thermal stress.
Hence a need exists in the photovoltaic industry for a low-bow, high-performance aluminum paste that forms a sufficient BSF layer in a solar cell contact, a method of making such a contact, and an Al paste that will also fire through a passivation layer.